ORGANIZATION IN THE MEMBRANE OF THE N-TERMINAL PROTON-TRANSLOCATING DOMAIN OF THE BETA-SUBUNIT OF THE PYRIDINE-NUCLEOTIDE TRANSHYDROGENASE OF ESCHERICHIA-COLI

The proton-translocating transmembrane pyridine nucleotide transhydrogenase of Escherichia coli is composed of two types of subunits, alpha and beta, The beta subunit has several membrane-spanning segments in the N-terminal region followed by a cytosolic C-terminal domain bearing a binding site for NADP(H), The N-terminal region contains at least one residue involved in the process of transmembrane proton translocation, Using site-directed mutagenesis cysteine residues were introduced at selected sites into the N-terminal region of the beta subunit, The pattern of labelling of these residues with 3-(N-maleimidyl propionyl)biocytin and other sulfhydryl reagents has shown that a model in which the N-terminal region of the beta subunit spans the membrane in eight segments is more likely than a previously proposed six segment model (Holmberg et al, (1994) Biochemistry 33, 7691-7700), The pl eferred model accounts for the site of labelling of a glutamate residue (Glu124) in the N-terminal domain by N,N'-dicyclohexyicarbodiimide. (C) 1995 Academic Press, Inc.